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Equity-based Awards, Restricted Unit Awards (Details) (Restricted Common Unit Awards [Member], USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting rate of restricted common unit awards (in hundredths) 25.00%   25.00%  
Summary of awards activity, equity instruments other than options [Roll Forward]        
Beginning of period (in units)     3,610,607  
Vested (in units)     (1,276,762)  
Forfeited (in units)     (115,250)  
End of period (in units) 2,218,595   2,218,595  
Restricted units outstanding, weighted-average grant date fair value [Roll Forward]        
Weighted-average grant date fair value per unit, at beginning of period (in dollars per unit)     $ 51.66 [1]  
Vested weighted-average grant date fair value per unit (in dollars per unit)     $ 47.81 [1]  
Forfeited weighted-average grant date fair value per unit (in dollars per unit)     $ 52.49 [1]  
Weighted-average grant date fair value per unit, at end of period (in dollars per unit) $ 53.84 [1]   $ 53.84 [1]  
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]        
Cash distributions paid to restricted common unitholders $ 1.6 $ 3.0 $ 4.1 $ 5.6
Total intrinsic value of our restricted common unit awards that vested during period 2.7 54.0 84.1 106.4
Unrecognized Compensation Expense [Abstract]        
Unrecognized compensation cost 47.1   47.1  
Recognition period for total unrecognized compensation cost     1 year 8 months 12 days  
Enterprise [Member]
        
Unrecognized Compensation Expense [Abstract]        
Unrecognized compensation cost $ 42.2   $ 42.2  
[1] Determined by dividing the aggregate grant date fair value of awards (before an allowance for forfeitures) by the number of awards issued.